Means for obtaining automatic volume control in radio sets



1934' L. F. WlLLGlNG 1,971,741

MEANS FOR OBTAINING AUTOMATIC VOLUME CONTROL IN RADIO SETS Filed Dec.16, 1930 [NV EN TOR.

A TTORNE Y.

Patented Aug. 28, 1934 MEANS FOR OBTAINING AUTOMATIC VOLUME CONTROL'INRADIO SETS Louis Willging, Cincinnati, Ohio, assignor to The CrosleyRadio Corporation, Cincinnati,

Ohio, a corporation of Ohio "Application December 16,1930, Serial No.502,737

' 40mins. (cram--) My invention is addressed to means which areautomatic in their action for controlling the volume in radio sets tothe extent of preventing the overload of parts and a consequentdiminution s or distortionin output. It is well understood that if theoutput of a radio tube is plotted against the inputflthe output willrise with input to a certain maximum beyond which further increases inthe input current strength will not produce correcw spending increasesin output. In some instances the output actually decreases with afurther input' increase; and distortion is introduced into the output ata point beyond, but very close to, the effective maximum output point onthe curve. The object of automatic volume control in radio sets isprimarily to prevent the input current from rising beyond the pointrepresented by the maximum value (without distortion) of the output forany given element, or for the set in gen- 2i) oral. This is ordinarilyaccomplished by some automatically actuated means which will cut downthe sensitivity of preceding tubes in the set so that the value of theinput current respecting some other tube or tubes, is prevented fromrisingjbeyond an allowable maximum. An element of the radio set may becaused to act as a control element and to supply a current which willaiiect the grid bias of preceding tubes to this end; and it isadvantageouaof course, to select as the controlling element, thatelement which is most likely-to become overloaded. For this reason it isadvantageous in a system of this charactcr to. make the .detector tubethe, controlling element and preceding radio frequency tubes thecontrolled elementa'in sets having radio frequency detector andamplifier stages. My invention is not limited in] this. way, however,and any tube in a set may, if desired, be caused to control theamplification characteristics of a preceding-tube 40 01" tubes.

An increase in the grid bias of a preceding tube should, of course, outdown the amplifying characteristic of that tube. In order that thesensitivity of the set may not be impaired upon weak 1& signals, orsignals which do not reach the allowable maximum, it is clearlypreferable to have volume control means which cut in sharply, at oraround the desired point of allowable input maximuzn, but increase veryrapidly in their action as iiilthe current rises beyond this point sothat the input to succeeding tubes or stages may be held substantiallyconstant at the highest permissible value.

An object of my invention is the production of "volume control means ofthe type generally re ferred to hereinabove which are simple andeffective in construction.

Another object'of my invention is the 'p'roduc tion of automatic volumecontrol whichis more readily controlled, togetherwith means to this end.

Still another object of my invention is the provision of means forvolume control in which it is certain that the incoming signal will becut down in strength prior to the point where distortion enters into theoutput current.

These and other objects of m'yinvention which will be set forthhereinafter, or will be apparent to one skilled in the artupon readingthese specifications, I accomplish by that certain construe tion andarrangement of parts of which I shall now describe a preferredembodiment, reference being had to the accompanying drawing, wherein Ihave shown in adiagrammatic manner, a circuit illustrating the essentialfeatures of my in-' 755? vention. The drawing is diagrammatic and theparts of a radio circuit not having to do directly with my volumecontrol means, have not been shown, since these parts are well known inthe art, and since they may vary from set to set, in-' asmuch as myvolume control means may be applied to sets of different character aswill here-' inafter appear. I

I have discovered, as set forth in my copending application, Serial No.455,353, filed May 24,1930, that at. or about the point of overload in atube, such as the detector tube of a radio set, the grid thereof beginsto draw current and that it is possible to impress this current backupon the grids of preceding tubes to vary the bias thereof. In 9 oneaspect my present invention is aneimprovement upon the invention setforth in the said application for Letters Patent, since I effect volumecontrol by a current drawn in a controlling element from a point betweenthe plate and the filament or electron emitting device thereof. Asrespects the invention of my said application, and its practicalapplication, I have found a very close correspondence between theoverload point in tubes and the point at which the grid begins to 160draw current, so much so that entire success has attended the control ofvolume by this means. But the overload point, the point of maximumvolume without distortion, and the point at which the grid begins todrawcurrent, vary somewhat in their relationship from tube to tube, and itis conceivable that in some tubes now or hereafter in use, the overloadand distortion points may be reached prior to the time when volumecontrol of 11 the type of my co-pending application, becomes entirelyeffective.

My present invention applies to screen grid or tetrode tubes, and itcontemplates a way of insuring that the grid bias of preceding tubes isincreased to effect volume control prior to the overload point. Thescreen grid of a tetrode tube also draws current after the input valueof the said tube has reached a certain maximum, but as will be readilyunderstood from a consideration of the nature of tetrode tubes, thescreen grid will begin to draw current in advance of a condition in thetube which would cause the grid itself to draw current. Current isdrawn, therefore, in the screen grid definitely prior to the overloadpoint of the tube and definitely prior to the introduction of distortioninto the tube output. I am thus able to effect volume control just assharply as before, but earlier. Due to the fixed characteristics of thetube, the point at which the screen grid draws current will always be inadvance of the point of overload and distortion, and in advance of thepoint at which the grid itself would draw current. The exercise ofvolume control from the screen grid potential does not, however, cutdown the maximum volume of the set by any appreciable amount.

In the drawing, I have shown a controlling tube 1, which may be thoughtof as a detector tube, although it may be another tube in the set. Thistube has a plate 2, a screen grid 3, a grid l, and an electron emittingdevice, or cathode 5. In this tube the screen grid current will increasewith the radio frequency voltage applied to the control grid. Thiscurrent increases rapidly near and before the overload point of thedetector, and this increase is what I have referred to herein as drawingcurrent. I have shown in the figure a high resistance element 6, inseries with the screen grid 3, and between it and a connection to theplate circuit of the tube. I have also shown a lead 7 connecting thescreen grid 3 with the grid 8 of a preceding tube or tubes 9. The tube 9may be a radio frequency tube also of the tetrode type. Its plate isshown at 10, and its screen grid at 11. 12 indicates its cathode. Thescreen grid of tube 1 maybe connected to the grid of tube 2 through anisolating resistance 13, in order to prevent coupling, and also throughsuch other instrumentalities as the input secondary 14 of the tube 9.The plate circuit of the several tubes is diagrammatically shown asconnected to a lead 15 of positive potential, and the cathodesdiagrammatically shown as connected to a lead 16. A stabilizingresistance 17 may be connected between the screen grid and the cathodecircuit of the tube 1. Its function is to help stabilize the screengrid. The voltage of the radio frequency cathodes, such as 12, may beadjusted so that with weak signals on the detector tube 1, the bias onthe radio frequency tubes, such as the tube 9, is correct for maximumsensitivity. An increase in the input signal value to the detector willincrease the current in the resistance 6, thus decreasing the screengrid voltage, which in turn, increases the bias on the radio frequencytube or tubes 9, thereby decreasing their amplification until a point ofbalance is reached.

In a typical circuit according to my invention, the element 13 has aresistance of one megohm, and the elements 6 and 1'7 have resistances of500,000 ohms each. The lead 16 is at ground or B potential and the lead15 has a potential of 300 volts, or the 13+ supply. In the detector tube1, the screen grid 3 is at a potential of about 50 volts above ground,while the cathode 5 is at a potential of about 6 volts above ground. Inthe tube 9, the screen grid is at a potential of about 150 volts aboveground, and the cathode of this tube is at about 53 volts above ground,being adjusted to give a net bias of about 3 volts on the grid 8.

It will be understood that modifications may be made in my inventionwithout departing from the spirit thereof.

Having thus described my invention, what I claim as new and desire tosecure by Letters Patent, is:

1. In a radio set having at least one tetrode tube and at least onepreceding tube, an ungrounded connection between the screen grid of saidtetrode tube and the grid of the preceding tube in said set.

2. In a radio set having at least one tetrode tube and at least onepreceding tube, an ungrounded connection between the screen grid of saidtetrode tube and the grid of the preceding tube in said set, and aresistance in series with said screen grid and a plate circuit of saidtetrode tube. 3:

control in a radio set comprising a tetrode tube and a preceding tubewhich consists in impressing back the screen grid variations ofpotential in the tetrode tube upon the grid of the preceding tube,thereby modifying the grid bias of said pre ceding tube in accordancewith the screen grid potential of the tetrode tube to control theamplification of the preceding tube in accordance with the value of thescreen grid potential of the tetrode tube.

LOUIS F. WILLGING.

